This invention relates generally to fuel burners and, more particularly, to fuel control systems for fuel burners.
Extensive efforts have been directed toward the improvement of fuel control systems for fuel burners such as gas and oil burners and the like. Increased system safety and reliability have been primary objectives of such efforts. These objectives, however, generally conflict with an obvious desire to limit the cost and physical size of the systems.
Most burner systems employ fuel supply valves that are automatically controlled by some type of flame sensing mechanism that automatically interrupts fuel flow in response to a predetermined loss of flame condition. According to one common technique, the presence of a flame is indicated by a signal current which is rectified by the flame as a result of the well known ionization phenomena. Although flame rectification provides a relatively effective method of sensing flame, prior systems of this type have suffered from certain disadvantages including the requirement for expensive isolation transformers for isolating the flame sensing circuitry from the power lines. Other problems of prior systems are associated with the necessity for isolating the d.c. flame rectification signal from a.c. component present therewith. In many poor flames the detection of directional conduction is marginal because of leakage in both directions and amplification does not fully solve the problem in that is is susceptible to a.c. pickup particularly when the amplifier is connected to the "hot" side of the line.
The above noted problems are avoided to some extent in the system disclosed in U.S. Pat. No. 3,441,356. In that system a single polarity supply is utilized to produce the flame responsive current and the relative conduction from a positive electrode is compared with that from a negative electrode to establish the presence of flame. However, in that type of system an inadvertent short circuit to the flame sensing electrode will produce a d.c. current that cannot be distinguished from a flame supported signal. Other common problems of this as well as other burner control systems are associated with the electronic elements used to monitor the signals produced by the falem rectification current. Typically, an electronic switching element such as a silicon controlled rectifier is gated by the flame signal to produce a desired control signal. False triggering of such conductor devices by transients is relatively common and reduces overall system reliability.
The object of this invention, therefore, is to provide an improved flame responsive control system for fuel burners that is both reliable and of reasonable cost.